Re: [ATM] Astigmatism on thin blanks

["David Harbour" <stainless_steel@suddenlink.net>]

>From another oldtimer:

About the issue of plate glass coming to thermal equilibrium; when it has 
finally cooled off, it can be diffraction limited in performance. I read 
where G. W. Ritchey waited with Hale, Adams, and others, for first light 
with the great Hooker 100"; when they finally looked at Jupiter, and then 
Vega, if my memory serves me correctly, the account said that Ritchey 
re-affirmed his prediction that the big glass would be a failure (it was 
made of plate, annealed under a pile of ungulate manure, by St. Gobain's)- 
and he left. He missed the second act, so to speak, that night.

But Hale abnd Adams, gripped by the idea that the mirror only needed to cool 
off, came stealing back for views that took their breath away in the wee 
hours. Thereafter, among other improvements, they made many innovations to 
try to keep the mirror, during the day time, at the predicted night time 
temperature (and rarely succeeded).

I enjoyed you post, "old timer".

ZRS-1

----- Original Message ----- 
From: <Starmutt@aol.com>
To: <atm@atmlist.net>
Sent: Thursday, May 22, 2008 7:19 PM
Subject: Re: [ATM] Astigmatism on thin blanks


>I  am an old timer that rarely contributes to the list but in this case I
> will. I  have made a few dozen mirrors and if you are careful and do your
> homework I find  the thin issue to be a non-issue. I have been lucky or 
> just careful
> and have  never had the issue with astigmatism. Thin has many advantages 
> that
> should not  be so easily dismissed due to fear or misgiving. Saving an 
> hour a
> night in  observing time is not a small issue. The main issue to me was 
> speed
> of the  mirror. If you want a super figure on a thin mirror it may be a
> problem  especially if you are new to the task. I would fall back to a 
> thicker
> mirror  depending on the need for a fast mirror. I would also indicate my
> preference for  good old Pyrex if making a thin mirror. The time needed to 
> figure the
> mirror if  made from plate glass in a thin scope is not a small affair. 
> Sure
> you can still  use the scope but individual need is a factor. For casual
> observing both thin  and plate glass may be acceptable. I tend to go for 
> Pyrex when
> making a thin  mirror and definitely fine annealing. Overall the addition 
> of
> cost to have the  glass annealed is a wise and almost necessary move. That 
> said
> I have several  fine 13" plate glass mirrors that produce a fine image and
> lots of hours of  enjoyment. Overall my favorite is still a thin 22" fine
> annealed Pyrex mirror at  f 5.25. Just a note, though I have been a silent 
> reader of
> the list I find it's  insights without equal. I have enjoyed the pro's and 
> the
> newbies and read every  word. The archives are prizes to be mined and most 
> of
> what people ask are well  covered somewhere in the past listings. Read a 
> lot
> and reread them again. Thanks  to all.
>
>
>
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